A large potential for small diameter spiral pipes exists in the filtration market, such as automotive oil and air filters, as well the HVAC market, such as insulated duct systems and silencers. These products typically have a perforated inner metal cylinder that is at least one inch diameter, and an outer cylinder mainly to support the filter medium, which is usually paper. Because pipes such as these need to be accurately and cleanly cut in large quantities, a forming and cutting apparatus is necessary. There are several known ways to form and cut a pipe. A pipe may be formed by spirally or helically by winding a continuous strip of metal, and joining adjacent edges of the wound strip to form a spiral lockseam in the pipe, as shown in U.S. Pat. No. 4,567,742. In some pipe forming and cutting machines, the spirally formed pipe is cut by moving a knife outside the pipe into an overlapping position with a knife inside the pipe. Other types of spiral pipe forming and cutting machines use multiple knives or rotate the knives around the pipe to cut the pipe into sections, as shown in U.S. Pat. No. 4,706,481.
The performance of the filter depends on the performance of the spiral pipe, typically an outlet at the center of the filter, where a strong flow of air or liquid is applied. A reliable and strong filter must be maintained to resist pressure and to insure functioning of the filter. An air filter consists of perforated inside and outside tubes with medium in-between. An end-cap closes one end of the filter, while the other end-cap closes the only medium surface, leaving a central area for inflow/outflow. The filter cleans by applying suction to the open-ended end-cap, drawing air through the filter medium, which retains debris.
Oil or liquid filters and separators typically have a solid outer tube and a perforated inner tube. The liquid to be filtered or separated is brought through one end between the outer tube and the medium. Under pressure, the liquid flows through the medium, which retains debris, and the liquid then flows through the perforated inner tube and leaves the filter. The filter element, or medium, is typically paper, but need not be, and may be made from any of a number of other materials.
In a double-wall HVAC system, the outer tube is solid and the inner tube is typically perforated. Insulation medium is inserted between the outer and inner tubes. The purpose of the medium is to reduce noise as well as heat transfer between the transported air and the outside environment. Silencers, made in a similar double-wall manner, are strategically placed into HAC ductwork systems to reduce noise. The perforations in the center pipe necessary for the filter to function may be achieved in several ways.
The strip or coil used for the central pipe may be perforated off-line, that is, in a separate operation. Of course, this requires separate operations for perforating the metal. Perforating off-line has some advantages, in that a stock of perforated sheet metal may be accumulated and stored for later use. This technique, however, also has several disadvantages. One disadvantage is that expanded coil is usually purchased from a vendor with expensive expanding machinery, and the price of expanded metal is thus expensive compared to coilstock. Another disadvantage is that inventories of perforated coilstock may tend to accumulate, driving up inventory and thus adding additional manufacturing cost. Another disadvantage is that perforated steel tends to rust. The longer the inventory is kept, the more severe the problem may become. What is needed is a way of perforating the coilstock in a “just-in-time” manner. Such a technique would avoid the accumulation of inventories of coilstock, would prevent inventories from deteriorating, would help to keep manufacturing costs low, and would eliminate dependence on expanded metal suppliers, with delivery and price variations.